Method of treating metal



May i9, 1936. A. R. STARGARDTER 2,041,029

METHOD OF TREATING METAL Filed Sept. 4, 1951 3 Sheets-Sheet 1 May 19, 1936- A. R. STARGARDTER 2,041,029

METHOD OF TREATING METAL Filed sept, 4, 1951 3 sheets-Sheet 2 l BNVENTOR AR. SOHQWcdP/C BY K/T it 69W@ WGRNEY METHOD OF TREATING METAL File@ sept. 4, 1951 3 Sheets-Sheet s ATTORNEY l Patented May 19, 1936 UNITED STATESl PATENT OFFICE Gillette Safety Razor Company,

Bolton, Mass..

a corporation of Delaware Application September 4, 1931, Serial No. 581,183

6 Claims.

An obiect` of my invention is to treat continuous lengths of magnetizable metal, such as hardened or annealed steel strips, (or ferrous-alloys) in such a way as to produce substantial uniformity of a desired character throughout the length of the strip, such as hardness and/or temper, or an annealed condition,v in a simple and relatively inexpensive manner.

Another object of my invention is to control the treatment of a metal strip as it travels in accordance with desired characteristics, such as hardness and/or temper or an annealed condition, of a piece of the same or a similar strip that has been hardened and/or tempered, or annealed, and which is used as a. standard, whereby the strip being treated will have substantially throughout characteristics similar to those of the standard piece and will be substantially free from variations from the latter.

Another object of my invention is to cause control of the heat applied to a. traveling steel strip by utilizing definite characteristics in portions of the strip, or variations or changes therefrom in the metallurgical characteristics within other portions of the strip itself, whereby to retain a substantially definite or predetermined heat applied to the strip in the heating Zone, if required, or to automatically increase or decrease the heat applied to the strip in such zone as may be required, as the strip travels, to cause the resulting product to be substantially uniform throughout its length according to a predetermined standard.

Any change in the microstructure of steel or any other ferrousalloy is accompanied by a change in the electrical permeability of the metal. For example, a steel which has been quenched from 1600" F. will possess less permeability than if it had been quenched from l500 F. The same steel quenched from 1500 1i. will possess less permeabiiity than if it had been quenched from 1400o F., at the same cooling rate.

l These phenomena are due to the fact that in a given steel or steels identical in composition, mi-v crostructure, and pre-natal history, the higher the quenching temperature the greater the amount of carbdes held in solid solution. The electrical permeability is inversely proportional to the amount of carbides in solid solution. This principle is also true for annealed steels.

A steel annealed from 1500 F. at a certain cooling rate will possess greater permeability than the same steel or steels identical in composition and pre-natal history annealed from 1400u F., since the steel annealed from 1500" will have less carbide in solid solution than that annealed from 1400 F.

It also follows from the above that in the case of either hardened or annealed steels (or ferrousalloys) that the electrical permeability is inverse- (ci. 14s-10) ly proportional to the cooling rate. Identical steels cooled at the rate of one degree per second will have greater permeability than those cooled at the rate of 40 per second, etc. Thus waterquenched steels have less permeability than sim- .ilar steels oil quenched.

It is also true that in the case of steels identical in composition and pre-natal history the electrical permeability varies inversely with the net carbon content.

In accordance with my invention, I utilize the electrical permeability of a standard piece of magnetizable metal which has been predetermined as to the desired characteristics to cause the strip to be heat-treated, namely, to be hardened and/or tempered, or annealed, in substantial correspondence as to desired characteristics in the treated strip of those in the standard strip.

In carrying out my invention, I provide an electrically lheated apparatus through which the strip to be heat-treated is propelled and electrical devices through which the heat-treated strip travels. Said devices are adapted to react to electrical permeability changes or variations in such strip as compared to a. standard piece of similar material in said devices to register, indicate and/or control automatically'any such changes or variations from the standard piece to cause the heating apparatus to apply a required temperature to a traveling strip, such as maintaining a given heat or reducing such heat, for a required period, and for restoring a higher heat as may be required, to cause the strip to be heattreated so that its desired characteristics will, throughout its length, substantially correspond to those of the standard piece.

In a particular embodiment of my invention as illustrated in the accompanying drawings, I provide an electrically heated induction furnace having an automatic control switch in its supply line, means to continuously propel a metal strip through the furnace, means to quench the heated strip, a pair of transformers respectively having passageways for the strip under treatment and for a piece of standard metal, the secondaries of the transformers having opposite windings electrically opposed, in an electrically balanced circuit including a galvanometer, a mirror operated by the galvanometer, a photo-electric cell to receive a light beam from the mirror,a lens system serving to focus the light emanating from the mirror upon the photo-electric cell window, and a circuit controlled by the photo-electric cell including devices to operate the aforesaid switch. The arrangement described is such that a standard metal piece will be placed in one transformer and the strip after emerging from the quenching and/or tempering means will travel 'through the other transformer. If the last named portion of the strip corresponds tothe standard piece the light beam from the galvanometer mirror will rest upon the photo-electric cell, causing the circuit through the furnace to remain closed for standard operation. Should the furnace temperature rise and change the then existing characteristics of the quenched strip a differential will occur in the circuit of the transformer secondaries causing the galvanometer to operate the mirror toremov'e the light beam from the photoelectric cell, thereby causing the latter to cease functioning and the circuit through the furnace will be broken. When the furnace drops to an appropriate temperature (and the characteristics of the quenched strip and the standard piece correspond) the galvanometer circuit will balance and the galvanometer will operate and cause the light beam to again fall on the cell to cause closing of the furnace switch to restore heating of the latter. In this way the temperature of the furnace dition as will produce a substantially uniform hardening of the strip throughout its length in substantial correspondence with the standard niece.

My invention also comprises novell details of improvement that will be more fully hereinafter set forth and then pointed out in the claims.

Reference is to be had to theV accompanying drawings, wherein Pig. l is a side elevation of an apparatus adapted to carry out my inventio Fig. 2 is a plan view of the apparatus;

Fig. 3 is a sectional view partially on line l, 3 in Fig. 2,

Fig. 4 is a cross section on line l, l in Fig. 3; g Fig. 5 is a detail view illustrating the galvanometer and photo-electric cell in operative relation;

Pigs. 6, 7, and 8 are detail views illustrating various positions of the galvanometer mirror with respect to the photo-electric cell, and

Fig. 9 is a diagrammatic view of the electrical -devices for controlling the furnace temperature.

Similar. numerals and letters of reference indicate corresponding parts in the several views.4

Upon a suitable frame I is mounted an electric induction furnace 2 through which a strip A of metal, such as relatively thin steel, is to be propelled in a continuous way. The furnace illustrated comprises a munie I of suitable metal, shown in tubular form, open at both ends for the passage of strip A. The muille is enclosed in insulation l around which is a conductor coil 5 that is enclosed in insulation i. Said parts are shown enclosed in an outer protective casing 1, (Figs. 3 and 4). The coil 5 is shown connected with conductors I and l leading from a high frequency converter Il of any desired construction, to be supplied from an A. C. line source il,

Il. Ihe muilie will be heated by induction from the coil I for heating the strip A assit travels. I'he coil l is shown in tubular form and cons nected with rubber hose at 5a, 5b, to be supplied with running water. cold water running through the tubular coil thus keeping the coil cool. Said strip may besupplied on a reel il and wound on a take-up reel Il in a well known way. Any suitable propelling means for strip A may be provided. I have illustrated a pair of opposed rolls Il and II suitably Journaled for gripping and feeding the strip. 'I'he roll Il is shown provided with a gear I'l in mesh with a wenn Il operative by chain il on sprockets 2l. 2l drivenvby a motor 22 to propel the strip A through the furnace at a substantially constant speed of travel.

may be maintained at sucha conp ondaries.

At Il and Il are cooling plates between which the heated strip A passes in a quenching zone from the furnace or heater for chilling or quenching the heated strip. A cooling water tank is indicated at 28 on plate 2l. 'I'he heated strip will be quenched in a well known way by passing between the chilling plates.

At 20 and 21 are a pair of step-down transfarmers having primary windings 28 and 29 in series in a circuit, shown including conductors a, b, and c, (Figs. 1 and 9). A switch at d permits connection of said conductors with the A. C. supply line conductors at e. An adjustable resistance is indicated at l for the conductor c. (Fig. 9). 'I'he secondary windings 28' and 28' of said transformers are oppositely wound and are thus electrically opposed, their terminals being shown connected by a conductor 30, the other terminals of the secondaries 2l' and 29 being respectively connected by conductors Il and 32 with the movable coil Il of alternating current galvanometer 34. 'I'he said secondaries have no permanent cores and the transformers are so constructed as to be in perfect electrical balance until strip A has passed through an electrical ileld in secondary 2l' and a standard metal piece B is placed to rest in an electrical field in secondary 2l'. serving as cores within the sec- Ihe primaries and secondaries have eways 2l" and 29", respectively, to receive the strip A and standard piece B. The circuit through coilll of the galvanometer field circuit is controlled by switch Il connected with the A. C'. supply line 3l to receive suitable electric current, (Flg.9) A mirror or reilector Il is connectedwith to be rotated thereby. At

` I2 is a light source, such as an electric lamp,

shownlocatedwithin thecasinglltocastalight beam on the mirror 4|. The lamp l2 is shown enclosed within a shield having an aperture u for the light beam. Different angular positions of the mirror, on one side or the other of a predetermined position, such as in Fig. 7, are utilized to indicate and/or control the flow of current through the coil l of the furnace in accordance with difference in characteristics between the standard piece B in transformer 2l and the portion of strip A passing through the transformer 20. 'l'he arrangement is such that when the mirror is in the position shown in FlgV'l, the circuit for coil l should be closed and if the temperature of the furnace should unduly rise, the strip A will be characteristics between the portion of the strip through the transformer 2l and the standard piece B to cause unbalancing of the circuit through the secondaries of the transformers. whereupon the coil 33 will rotate the mirror to an angular position at one side of the position of Fig. 7, such as to the position shown in Pig. 6, and the circuit for the coil I will be broken automatically. When the temperature of the furnace has dropped so that the characteristics of the portion of the strip through the transformer 2l compares with the characteristics of the standard galvanometer coil 33 will operate to mirror to its normal position. such as in Pig. 7, whereupon the circuit through the coil I will be closed automatically.

lf a still greater divergence in the metallurgical characteristics of the pre-treated strip should exso heated astccause a difference inV farther to the left (to position snown'in rig. si, and the furnace heating circuit will remain closed until the traveling strip, due to this heating, will The circuit 4s ofthe photo-electric een through..

the amplifying .tube 41 is utilized to control the ilow of current through the coil 5 of the furnace by the operation of a relay or magnetic,

contactor 48 to control the A. C. supply line I2. y

(Fig. 9). To obtain the desired power to operate the magnetic contactor', I vhave illustrated a transformer 49 whose secondary winding 5I! is included'in the circuit 5I of the photo-electric cell and whose primary winding 52 is supplied with suitable A. C. current through the-switch controlled line 53. A grid-leak g, a fixed condenser h and a potentiometer i areshown for the photoelectric cell circuits.` A relay 54, as controlled by a solenoid' coil 55 in the circuit' 5I of the photoelectric cell, may be normally kept open by a spring 56. When the light'beam is upon the photo-electric cell, suicientpower from photoelectric cell, amplified through the system shown,

will energize coil 55 to cause the relay 54 to close the line 51 through the coil 58 of the normally Open relay contactor 48 to cause the latter to close the circuit of line II, I2 to the converter to supply current to coil 5 of the furnace. The coil 58 may be a solenoid coil, the contactar being normally held open by a spring 59, or by its own weight. l

In order to assure that the light beam from mirror 4I will not'affect the photo-electric cell 45 when the mirror is in certain positions but will affect said cell when the mirror is in other positions, I provide the casing with a front wall 4l' and with a front opening 40", as indicated in Figs. 6, 7, and 8. When the mirror is .in the position shown in Fig. 6, the light beams or rays from the mirror will be obstructed by the wall 40 so as no t to affect the cell 45, and when the mirror has moved a suitable distance, suchas to the positions shown in Figs. 7 and 8, the light` beam may be cast upon the cell through the opening 40" of the casing 4II. In order to assure the concentration of the light beams or rays on the photo-electric cell in all appropriate positions of mirror 4|, I interpose lenses at 60 and 5I between the mirror and the cell to intercept the beam or light rays from the mirror and rectify them so that they will be directed upon the cell in different positions of the mirror from the position of the latter shown in Fig. 6 to its positions shown in Figs. '7 and 8. The lenses are shown` connected together by an interposed tube 52 through which the rays may pass from 4the mirror.

The tube B2 may be supported by casing 40 by means of a post 63, (Fig. 5). The lens 60 is shown having its -convex surface opposing the mirror and the lens 6I has its convex surface opposing the photo-electric cell, whereby in different anguiar positions ofthe mirror, with respect to the lenses, the light beam or rays from the mirror willbe caused to be directed against the cell, converging to a point at the cell Window.

The operationy may be described as follows: Two pieces of strip metal of similar manufacture to that' of the strip A to be treated and bearing the structure desired to be obtained through the treatment of strip A may be placed in the passageways of the transformers `25 and 21. The primary coils of the transformers and the galvanometer ileld circuit are then closed atthe switches d and 31 to energize the circuits, and resistance f-will be adjusted until the mirror 4I assumes a position to cast the beam uponthe photo-electric cell,

such as indicated in Fig. '1, for causing closing of' the respective circuits at therelays 54 and 48, whereby current will be caused to flow through the coil 5 of the furnace for heating the latter.

The standard piece B is allowed to remain in the transformer 21, the other piece is removed from the transformer 25 and the strip A to be treated extends from the furnace andis inserted in the transformer 25 and passed through the rolls. and I6 to the reel I4. Assuming that the standard piece B has been quenched from 1500 F. and 'it is desired to have the strip A heated to 1500 F. as it travels, the motor 22.wi1l be caused to operate to propel strip A when the temperature of 'the furnace has reached the. desired degree,

`such as 1500 F. So long as the furnace. temf perature remains constant and the strip A is heated and quenched as it travels to correspond in desired characteristics to'those of the standard strip B, the circuit of the primary windings 2Q and 29 and the bucked circuit `of the secondaries of lthe transformers will remain balanced and. the galvanometer will cause the mirror to be in such a position as to cast the light beam through the lenses upon the photo-electric cell to maintain the. relays,v closed at 54 and- 48 to `continue the flow of current through the furnace coil 5. At such time, the quenched strip A passing through the secondary 28 and the standar piece B in secondary 29' correspond in the de sired characteristics. Should the traveling strip A, at some portion after quenching, as it passes through said secondary, vary from that of the standard piece, such as to require less heat, a' differential will occur in the circuit of the secondaries of the transformers, because of different characteristics between Vthe standard piece and the said portion of thestrip, thereby causing operaf tion of the galvanometer toactuate the mirror and move the light beam from the photo-electric cell, as shown in Fig. 6,. Thereupon the photoelectric cell'circuit controllingvthe coil 55 will.

vcease functioning and the spring will causel the contactor 54 to break. the circuitV 51 whereupon the spring 58 will operate the contactor 48 to break the circuit of the line I I, I2 and the cur- .rent will be broken from the furnace coil 5. The

traveling strip A will continue to be heated by the remaining heat in the' furnace to bring the strip substantially to the characteristics of the standard piece. When the net temperature of the furnace drops, say 10 F., and the characteristics of the strip A correspond to the characteristics of the standard piece B, the circuit of the transformer secondaries-will again be balanced andA the galvanometerwill operate the mirror 4I to cause `the light ray to fall on the photo-,electric cell to cause closing of the contactor 54 at the circuit 51, whereupon the coil58 will be energized and cause closing of the contactor 48 of the line circuit I I, I2 to restore the current to the furnace coil 5 to continue heating of the letter. The operations 'described occur successively as variations in characteristics occur in the heated and quenched strip with respect .to the characteristics of the standard piece B for required treatnntofthestripAasittravels. -Ifthe furnacetemperatureshouldriseabovetheproper degree oftheheatedandquenchedstripAaseompared tothestandardptecenthecircuitofthesecondaries 2l' and Il' through the coil 33 of the galvanometer will be so changed sato cause operation of the mirror Il to remove its light beam fromthephoto-electric cell, (Fig.6), thereby deenergizing coil 5l to cause break of circuit l1 tocause consequent breakingofcircuitatswitch u and shutting olf of the current through coil ofthefurnace until such-timeasthe furnace suiilcientiy cools to cause the quenched strip A to again correspond with the standard piece B, thereupon the circuit of thethrough coilwillbeagalnbalanoedandcausethemirror tocastalightbeamonceillltocauseciosingof circuits at contactors 5I and Il and to restore current to coil 5. Should the furnace temperature remain absolutely constant, any structural differences inherent in the strip will become apparent and will be automatically compensated for to produce a uniform product. For example, if portions of the hardened strip contain less carbide in solid solution, the hardening temperature applied to portions of strip A richer in dissolved carbide will yield a quenched product higher in permeability when applied to the portion leaner in dissolved carbide, so that when such portion of the strip A (e. g. leaner in dissolved carbide in the pre-hardened state) passes through the transformer 26 after being quenched at the prevailing temperature, it will possess greater permeability thanthe strip higher in dissolved carbides, and will cause the galvanometer light beam to be turned to fall on the photo-electric cell, as though the furnace temperature had fallen, or, in other words, as though the pre-hardened strip richer in dissolved carbide had been quenched from a lower temperature. The action of the photoelectric cell with the light beam upon it, will cause the operation of the contactors 54 and Il to cause current to flow through the furnace coil 5 until that part of the strip A leaner in dissolved carbides passes through the furnace and upon arrival in the transformer 25 of the strip richer in dissolved carbides in the original state, the pervmeability of the quenched strip will be again decreased, and the galvanometer mirror will operate tocausethelightbeam topassfromthephotoelectric cell, thereby causing operation of the contactors 5I and Il to break their circuits, to shut olf the current ilow to coil 5, to cause automatic cooling of the furnace, to compensate for the changed characteristics of the strip A being treated. The furnace temperature will be constantly altered to meet any changes in the continuity of the metallurgical characteristics within the steel strip A as compared to the standard piece B. The resulting product will be substantially uniform since the hardening temperatures will have been controlled so as to produce a substantially uniform amount of carbide in solution in the steel strip A. The automatic temperature changes described may be secured quickly enough to maintain a high degree of uniformity within the strip A which constantly emerges from the hardening system, through use of a high-frequency induction furnace, which is unusually adaptable to rapid temperature alteration.

While I have described the construction and the operation as applied to hardening and chilling a strip A of steel, it will be understood that my invention may beutilised for tempering hardened andtherebychangethcchlracteristics steel strips, as well as controlling annealing of ferrous metal strips, and quenching rates of hardened steel, and also for controlling the speed of travel of a strip through a machine of the character described. In the drawings, I have illus trated means for tempering the hardened and quenched strip as it travels from the furnace described. The furnace 2 having a mulile 3 and conductor coil iconnected with line conductors I- and 9', similar in construction to that described with respect to the furnace 2, is shown mounted on frame I, beyond the transformer 26, in alignment with said transformer, whereby the hard cned and quenched strip A from furnace 2 will travel through the furnace 2 to be heatedto the desired extent for tempering the hardened strip. At a suitable distance from the furnace 2 for cooling the strip A in its travel is a transformer 25 for passage of said strip, and at 21' is a transformer to receive a "standard tempered piece B. A galvanometer and mirror similar to those previ- .ously described is within a casing ll' and conductors 3i' and 32 connect the movable coil of the galvanometer with the transformers 25' and 21' in manner previously described respecting corresponding parts. A photoelectrlc cell l5' is in position to receive a light beam from the mirror in casing 40'. An amplifier Il and transformer Il', similar to the amplifier Il and transformer I9, having the supply lines 53', and the conductors 46' and 5i' of the photo-electric cell circuit, corresponding to the parts previously described, will control the circuit through the conductors 8' and 8' for the coil 5' in manner previously described, and conductors at 38 supply the gaivanometer field. Switches and relays (such as d, 31, 48, and 54) for the corresponding circuits, as previously described, including means, such as solenoids like 58 and 55 to operate the associate contactors, together with other adjuncts previously set forth, will be supplied to complete the electric devices, whereby a. normally balanced opposed circuit between the secondary windings of the transformers 26' and 21 will be established.

A standard piece B', previously hardened and tempered as desired, will be placed in the passageway of the transformer 21' to serve as a core therein, and the hardened and quenched strip will be passed through the munie 3 and through the passageway of the transformer 25', as illustrated in Fig. 1. Suitable high-frequency electric current will be supplied to the coll 5' to cause the mufiie 3' to be heated by induction to the desired degree. The operation of the galvanometer and its mirror in association with the photo-electric cell I5', will be similar to that previously described, in connection with the bucked circuit between the secondares of the transformers 26' and 21' for causing control of the flow of current through coil 5' for maintaining the required temperature of the munie 3', in accordance with the characteristics of the tempered strip A passing from the furnace 2' with respect to the characteristics of the standard piece B. That is to say, `so long as the characteristics cf the tempered portion of the strip A in the transformer 25 correspond to the characteristics of the standard piece B', the corresponding galvanometer mirror will be in position to maintain a beam on the photo-electric cell I5' to. keep the muiiie 3' at a desired temperature for standard operation, Should the temperature of the muilie 3' rise and change the then existing characteristics of the adjacent portion of the tempered strip A, a differential will occur in the circuit of the secondaries of the transformers 26' and 21', causing the associate galvanometer to operate its mirror to remove the light beam from said cell, thereby causing the latter to cease functioning and the circuit to the coil will be voperations described follow in succession as the tempered strip travels to maintain the tempering of the strip in substantial accord with the desired characteristics of the tempered standard piece B.

in accordance with the arrangements set forth, a. traveling steel' strip may be hardened and quenched, and then tempered, successively in a vcontinuous manner, and the finished tempered strip may be wound on the reel I4 in an expeditious and economical way, with the assurance that the hardening and the tempering of the strip will be throughout its length in accordance with the standards of hardness and tempering desired to have characteristics of hardness and temper substantially correspond to those of thestandard pieces selected, such as B and B'.

While I have illustrated and described means for hardening and subsequently tempering a con'- tinuous strip of metal as it travels, it will be understood that a strip may be hardened without at the same time tempering it, or that a strip previously hardened elsewhere may be tempered by passing through a furnace 2. An apparatus may be provided with a. hardening furnace or heater and its adjuncts as described or with a tempering furnace or heater and its adjuncts described. It also will be understood that a strip may be passed through either of the furnaces or heaters described for annealing the strip in` conjunction with a standard annealed piece in an appropriate transformer, .whereby the circuit through the secondaries of two transformers ldescribed will be controlled for controlling the temperature of the annealing furnace or heater in manner described with respect to hardening or tempering a metal strip. The invention also may be utilized to control quenching rates, or the speed of the metal strip in its travel throughthe heattreating devices, or in any other way, which will constantly insure the continuous. agreement of the strip being treated with the standard adopted. By the means described, the furnace may be quickly heated to the desired temperature and may be maintained approximately at such temperature because the supply current for the furnace may be turned on and off automatically in accordance with the correspondence of desired characteristics between the standard piece B or B' selected and the. portion of the strip being treated, since the current flow will continue so long as the proper heat is applied to the strip in accordance with its electrical permeability or the characteristics utilized. When such permeability or characteristics vary from that of the standard piece in such a manner as to ailect the desired treatment of the strip the current now will be automatically turned off and on, as required. and the furnace control may thereby be secured automatically, and the temperature of tbe furnacemey be quickly altered to meet any changes in the continuity of metallurgical characteristics within the steel strip. It is distinguished' from the previous practice of furnace control from an arbitrary, pre-determined temperature maintained by stationary thermo-cou- 5 ples within the furnace. The resulting product, according to my invention, is substantially uniform since the hardening or the tempering may be so controlled as to constantly produce a uniform amount of carbide in solution in a traveling steel strip, whereby to eiect the desired results therein. This is eected because the automatic temperature changes described may be secured quickly enough to maintain a high degree or". uniformity within the strip as it continues to emerge from the hardening and/or tempering system.

Having now described my invention, what l 'claim is:-

l. 'lihe method or heat-treating a ferrous metal strip consisting in continuously passing the strip successively through a heated zone and through a quenching zone, passing the heated and quenched portion of the strip through a balanced electrical field and varying the temperature of the heated zone to cause the required amount of heat to be applied to the strip as it travels through said zone by means of variations in the electrical permeability of said strip in said held.

2. The method of heat-treating a ferrous metal 30 strip consisting in passing the strip through a heated zone to heat the strip, passing the strip through a' quenching zone, passing the strip thereafter through electrically operative devices, and causing by means of a standard strip in said 35 devices operation of the said devices to control the temperature of the heating zone.

3. The method of heating and quenching a ferrous metal strip consisting in passing the strip through a heating zone and through a. quenching zone and varying the temperature of said heating zone in accordance with the variations in the electrical permeability of the quenched strip.

4. The method of hardening and tempering a ferrous metal strip consisting in passing the strip successively through a heating zone, a quenching zone and another heating zone, and varying the temperature of the first heating zone as the strip travels through said zone in accordance with the variations in the electrical permeability of said stripafter being heated and quenched.

5. The method of hardening and tempering a. ferrous metal strip consisting in passing the strip successively through a heating zone, a quenching zone, and another heating zone for tempering the strip, varying the temperature of the first named heating zone in accordance with the variations in the electrical permeability of the quenched strip, and'varying the temperature of the second heating zone in accordance with the variations in the electrical permeability of the tempered strip.

6. The method of hardening and tempering a ferrous metal strip consisting in passing the strip 

